Region-dependent effects of flibanserin and buspirone on adenylyl cyclase activity in the human brain

The mode of action of antidepressant drugs may be related to mechanisms of receptor adaptation, involving overall the serotonin 1A (5-HT1A) receptor subtype. However, so far, the clinical effectiveness of selective compounds acting at this level has proved disappointing. This could be explained by the heterogeneity of 5-HT1A receptors within the central nervous system. In animals, two 5-HT1A agonists, flibanserin and buspirone, have shown different pharmacological properties, depending on the brain region. Since no evidence supports this observation in humans, this study sought to investigate whether these two drugs exert different effects on 5-HT1A receptor activation in three different human brain areas: the prefrontal cortex, hippocampus and raphe nuclei. 5-HT1A-mediated inhibition of forskolin-stimulated adenylyl cyclase (AC) was taken as an index of 5-HT1A receptor activation. Flibanserin significantly reduced the activity of AC post-synaptically, i.e. in the prefrontal cortex [EC50 (mean +/- S.E.M.), 28 +/- 10.2 nM; Emax, 18 +/- 2.3%] and in the hippocampus (EC50, 3.5 +/- 3.1 nM; Emax, 20 +/- 4.0%), but had no effect in the raphe nuclei, i.e. at pre-synaptic level. Vice versa, buspirone was only slightly but significantly effective in the raphe (EC50, 3.0 +/- 2.8 nM; Emax, 12 +/- 1.9%). Agonist effects were sensitive to the 5-HT1A antagonists WAY-100135 and pindobind 5-HT1A in the cortex and raphe nuclei, whereas buspirone antagonized flibanserin in the hippocampus. These findings suggest a region-related action of flibanserin and buspirone on forskolin-stimulated AC activity in human brain.